The present invention relates generally to dishwashing machines and more particularly, to methods and apparatus for controlling the heating of a cleansing liquid held within a tank in the machine, the liquid being used for spraying upon ware to be cleaned.
Most dishwashers operate by holding ware to be cleaned within an essentially enclosed washing chamber. A cleansing liquid, most often water having a detergent added thereto, is contained within a tank until washing commences. The water is pumped from the tank into a spray system, from which it is sprayed into the chamber and onto the ware to flush soil from the ware. As the water drains, it is directed back to the tank for recirculation.
Because heated water cleanses ware more effectively and is required for proper sanitization of the ware, the water used within the dishwashing machine is typically at a relatively high temperature at the time it initially enters the machine. In order to keep the water at a sufficiently high temperature, a heater is typically mounted within tne tank, and is energized as needed to maintain water temperature.
Sanitization considerations require not only that ware items be heated through contact with heated water at certain specified minimum temperatures, but also that the items be maintained at such temperatures for certain minimum times. Consequently, not only is water temperature important, but the quantity of such water that is sprayed on the ware and the duration of such spraying are also prime considerations. However, these concerns conflict with energy consumption considerations in that heating of water represents a substantial source of energy usage within the machine. Accordingly, it is advantageous to hold water temperatures and durations of exposure of the ware as close to the minimum for adequate cleaning and sanitation as is possible.
The foregoing is particularly important in conveyor-type commercial dishwashing machines. In single-rack type commercial machines or domestic machines, washing is typically carried out by maintaining the ware in a stationary position while it is sprayed with cleansing liquid. In such a case, it is a relatively minor matter to pump water into the spray system for a slightly longer time than the minimum. On the other hand, in a conveyor-type machine, the ware is continuously moving through the machine while it is sprayed with liquid. Thus, the ware can be sprayed only as long as it is adjacent to the spray manifolds, and the quantity of water which is sprayed onto the ware must therefore be great enough to ensure sanitation. Even so, to avoid heating greater quantities of water than needed, thereby wasting energy, it is very important that any water that is sprayed onto the ware be at the proper temperature.
What is needed, therefore, is a control system and method for a tank heater which provides an effective compromise between the competing concerns of ensuring sanitation and holding energy consumption to a minimum. Such a system should be compatible with existing overall dishwasher control systems, and should be capable of incorporation into a variety of overall control system designs.